The use of ultrasonic imaging for medical diagnostic purposes is well-known. In particular, ultrasound has been used for over twenty years to aid in the diagnosis of certain cardiac diseases. Recently, cardiac Doppler ultrasound technology has become recognized as an important tool in the evaluation of cardiac blood flow rates. In Doppler ultrasound imaging, a reflection from a stationary object provides a signal at zero frequency (that is, at the intermediate frequency). The Doppler frequency shift in the echo signal returned from a moving target, though, varies monotonically with the instantaneous velocity of the target. A good, but brief, review of cardiac Doppler measurement technology is contained in R.G. O'Connell, Jr., "The Role of Doppler Ultrasound in Cardiac Diagnosis," Hewlett-Packard Journal, June 1986 at 20-25; in P.A. Magnon, "Doppler Effect: History and Theory," id. at 26-31; in L.I. Halberg et al, "Extraction of Blood Flow Information Using Doppler-Shifted Ultrasound," id. at 35-40; and in B.F. Hunt et al, "Digital Processing Chain for a Doppler Ultrasound Subsystem," id. at 45-48. As stated in the O'Connell article, there are two important aspects to the Doppler equation which impose limitations where the evaluation of cardiac disease is concerned. The first apsect is the angle between the flow velocity of interest and the incident ultrasound beam. The most accurate velocities are measured when the angle is very small. However, when searching for certain cardiac anomalies, such as high-velocity jets caused by stenotic, regurgitant, or shunt lesions, or defects in the heart, the exact angle of flow is unknown, and movement or rotation of the transducer is necessary until the location of the highest maximum velocity is obtained. The other important aspect of the equation is the proportional relationship between the frequency used to interrogate the blood flow and the resultant frequency shift. Due to this relationship, both pulsed Doppler and continuous-wave (CW) Doppler measurements are often employed.
A typical prior art medical ultrasound imaging system employs a phased array transducer, a scanner unit and a signal processing and display unit. The scanner unit provides analog signal conditioning, beam forming and signal translation from the ultrasound range to a more convenient intermediate frequency (I.F.) range. (The details of a typical beam forming operation and scanning function are discussed in S.M. Karp, "Modifying an Ultrasound Imaging Scanner for Doppler Measurements," Hewlett-Packard Journal, June 1986 at 41-44). The processing and display unit then converts the analog I.F. signals to digital form and processes the digital samples in order to facilitate extraction and display of desired information contained in the transducer output. The display and processing unit may provide both black and white (monochrome) as well as color imaging. The monochrome mode typically is used to show anatomic detail, with blood flow shown in the color mode. In a typical system, a two-dimensionsal monochrome image may show a sector- (i.e., arcuately-) shaped scan region (i.e., volume) of a patient, displayed at a rate of approximately 30 frames per second. A color mode image may be overlaid on a portion (up to 100%) of the scanned sector, displacing the monochrome image. At each picture element on the display, either the monochrome signal or the color signal is displayed; alternatively, the two signals may be combined in some fashion.
The color image is typically a color-coded blood flow map, where the color coding indicates localized velocity and tubulence of blood flow. In an exemplary commercial system, velocity is shown in shades of red and blue, red indicating flow toward the transducer and blue indicating flow away from the transducer, or vice versa; sometimes another color may be mixed in over a portion of the scale, to focus attention on flows within selected ranges. The intensity and/or shading of the color represents the speed of the flow toward or away from the transducer. Shades of green are sometimes added to indicate turbulence.
Velocity is measured using Doppler frequency shift techniques, which are well known. Turbulence is calculated, based on sample-to-sample consistency of velocities. Unfortunately, the received (i.e., echoed) signal at the transducer output contains not only a Doppler shift component due to reflection from the moving blood, but also Doppler components due to reflections from the motion of tissue structures such as blood vessels, heart walls and valves. Most significantly, since the heart wall is constantly in motion and is denser than the blood, it contributes a substantial Doppler signal which is significantly larger in amplitude (but generally lower in frequency) than the signal generated by the blood flow itself. A primary function of the signal processing and display unit is, therefore, to separate to the extent possible the signal due to the blood flow from other extraneous signals, such as those due to heart wall motion (These extraneous signals may be termed "clutter.")
Separation of the blood flow signal from the clutter is achieved with a clutter rejection filter and a velocity sample rejection system. The clutter rejection filter provides a frequency-dependent attenuation (or gain) of the received (i.e., returned echo) Doppler signal; the gain is higher for the blood flow signals (which are higher in frequency since blood flow is higher in velocity) than for the clutter signals. After the received signal has been thus filtered, it is sampled and velocity calculations are made from samples. Each computed velocity value is then "screened" against certain rejection (i.e., validation) criteria by the velocity sample rejection system. Velocities which have been determined from samples whose amplitudes (or at least one of whose amplitudes) are (is) below a predetermined acceptance/rejection threshold are considered unreliable and are therefore "discarded" by the velocity sample rejection system (i.e., they are neither displayed nor used in further calculations).